1. Field of the Invention
The present invention relates to microwave coupling networks, and more particularly to such networks used in the formation of multiple transmission beams from a circular radar or communications array.
2. Description of the Prior Art
It is well known in the prior art that phased arrays can be steered in angle by varying the phase of the drive on each element of the antenna array Such phasing is commonly performed with electronic phase-shifters in linear and planar arrays and with a combination of switching and phase-shifters in circular arrays. well-known method of steering a beam transmitted from an array of emitters uses a coupler network to control the phase of the signal sent to the emitters in step increments by switching an input signal among several inputs of the coupler network. The method is most applicable to planar or linear arrays rather than the circular arrays addressed here. It is also well known in the prior art to steer linear or planar arrays utilizing passive elements.
The Butler Matrix is also a well-known example of a coupler network in the prior art for achieving the effect of steering transmitted beams in planar and linear arrays. When applied to circular arrays, however, the Butler Matrix must be utilized with a multiplicity of variable phase and fixed phase shifters in combination with a power divider in order to achieve both phase and amplitude distributions. Typical Butler Matricies use 3 dB couplers for uniform illumination and one beamwidth steps. The Butler Matrix switching method for circular arrays is an extremely complex system requiring costly and potentially lossy phase shifters.
The Butler Matrix operated in conjunction with active phase shifters is a well-known method in the prior art to form and steer circular and cylindrical arrays. The U.S. Pat. No. 4,316,192, issued Feb. 16, 1982, to J. H. Acoraci entitled, "Beam Forming Network for Butler Matrix Fed Circular Array"; the U.S. Pat. No. 4,414,550, issued Nov. 8, 1983, to C. P. Tresselt entitled, "Low Profile Circular Array Antenna and Microstrip Elements Therefor"; the U.S. Pat. No. 4,425,567, issued Jan. 10, 1984, to C. P. Tresselt entitled, "Beam Forming Network for Circular Array Antennas"; and, U.S. Pat. No. 4,639,732, issued Jan. 27, 1987, to J. H. Acoraci et al. entitled, "Integral Monitor System for Circular Phased Array Antenna," all describe and claim the signal forming and steering technique.
Circular arrays are more difficult to steer than planar linear arrays, because both the phase and amplitude must be controlled to achieve steering. Neither phase nor amplitude varies linearly.
The problem to be solved therefore is the development of a direct method of forming multiple orthogonal beams from a standard circular or cylindrical array, wherein one or more beams may be formed simultaneously or switched around the circle or a sector of the circle of the array.